lib/src/: a2i_strto[iu](): Support other errno values

An implementation of strto[iu]max(3) may choose to fail with for example
ENOMEM.  Behave well with those.

The reason for the weird indentation is to reduce the diff in the
following patch.

Reported-by: Bruno Haible <bruno@clisp.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>